Cell Biology: Intracellular Transport, Cell Signaling, and the Cytoskeleton

Comprehensive vocabulary flashcards covering intracellular transport, cell-to-cell signaling, signal transduction cascades, and cytoskeletal filaments from biology lectures.

Nuclear Envelope

A double-membrane barrier enclosing the nucleus that consists of an inner nuclear membrane, an outer nuclear membrane continuous with the endoplasmic reticulum, and nuclear pores.

Nuclear Lamina

A mesh-like network of intermediate filament proteins (lamins) lining the inner face of the inner nuclear membrane that provides structural support and attachment sites for chromatin.

Nuclear Pore Complex (NPC)

A massive multiprotein assembly containing $50\text{--}100$ polypeptides with an overall complex diameter of $120\text{ nm}$ and a functional opening of $25\text{ nm}$ that mediates selective bidirectional traffic between the nucleus and cytosol.

Nuclear Localization Signal (NLS)

A short targeting sequence rich in positively charged amino acids (such as lysine and arginine) that remains uncleaved and directs folded cargo proteins into the nucleus.

Importin

A soluble nuclear import receptor protein that binds specifically to the nuclear localization signal (NLS) of cargo proteins in the cytosol to facilitate their translocation through nuclear pores.

Ran GTPase

A monomeric GTP-binding protein whose unequal nuclear and cytoplasmic nucleotide states drive the directional transport of macromolecules into and out of the nucleus.

RanGAP

A cytoplasmic GTPase-activating protein that triggers Ran to hydrolyze bound GTP to GDP, leading to the dissociation and release of importin in the cytosol.

RanGEF

A nuclear guanine nucleotide exchange factor that catalyzes the exchange of GDP for GTP on Ran inside the nucleus, maintaining a high concentration of Ran-GTP.

$$\text{I}\kappa\text{B}$$

An inhibitory cytoplasmic protein that masks the nuclear localization signal of the transcription factor $\text{NF}-\kappa\text{B}$; its phosphorylation and subsequent destruction release $\text{NF}-\kappa\text{B}$ for nuclear translocation.

Signal Recognition Particle (SRP)

A cytosolic ribonucleoprotein complex that binds the hydrophobic ER signal sequence as it emerges from a translating ribosome, temporarily halting translation until docking at an SRP receptor on the ER membrane.

Protein Translocator

A membrane-bound protein channel that receives nascent polypeptides targeted to the ER and transfers them across or into the endoplasmic reticulum lipid bilayer.

Signal Peptidase

An ER lumenal enzyme that cleaves the N-terminal hydrophobic signal sequence from translocated polypeptide chains to release mature soluble proteins into the ER lumen.

Rough Endoplasmic Reticulum

A region of the endoplasmic reticulum studded with ribosomes on its cytosolic surface, dedicated to the synthesis, folding, disulfide bond formation, and initial glycosylation of secreted, transmembrane, and lysosomal proteins.

Smooth Endoplasmic Reticulum

The ribosome-free region of the endoplasmic reticulum that functions as the primary site of cellular lipid synthesis and membrane construction.

Golgi Apparatus

A polarized collection of flattened membrane-bound cisternae (cis, medial, and trans) that modifies oligosaccharides, processes, and packages proteins received from the ER for delivery to their final cellular destinations.

Constitutive Exocytosis

An unregulated, continuous pathway present in all eukaryotic cells that supplies newly synthesized lipids and proteins to the plasma membrane and secretes extracellular matrix components via bulk flow.

Regulated Exocytosis

A specialized secretory pathway in cells such as endocrine cells and neurons where proteins are packaged into concentrated secretory vesicles and released only upon receiving an external signal.

Endocrine Signaling

A long-distance mode of cell communication in which specialized cells secrete hormones directly into the bloodstream to reach distant target cells bearing matching receptors.

Paracrine Signaling

A form of local cell communication in which signaling molecules diffuse through the extracellular fluid to act on neighboring cells without entering the bloodstream.

Autocrine Signaling

A specialized form of paracrine signaling wherein a cell responds to local chemical messengers that it secretes itself.

Juxtacrine Signaling

Contact-dependent signaling requiring physical interaction between membrane-bound signal proteins on one cell and transmembrane receptor proteins on an adjacent cell.

Chemical Synapse

A specialized junction where an electrical action potential in a presynaptic neuron triggers calcium-dependent exocytosis of neurotransmitters across a narrow cleft ($20\text{--}40\text{ nm}$) to bind postsynaptic receptors.

Excitatory Postsynaptic Potential (EPSP)

A local depolarization of the postsynaptic membrane typically caused by the opening of ligand-gated cation channels (such as $\text{Na}^+$ influx), driving the membrane potential closer to threshold.

Inhibitory Postsynaptic Potential (IPSP)

A local hyperpolarization of the postsynaptic membrane commonly generated by ligand-gated $\text{Cl}^-$ influx or $\text{K}^+$ efflux, decreasing the likelihood of generating an action potential.

Acetylcholine

A prominent neurotransmitter that triggers skeletal muscle contraction via ligand-gated ion channels and regulates autonomic parasympathetic responses via G-protein-coupled receptors.

Acetylcholinesterase

An enzyme located in the synaptic cleft of the neuromuscular junction that hydrolyzes acetylcholine to rapidly terminate the synaptic signal.

Glutamate

The primary excitatory amino acid neurotransmitter in the vertebrate central nervous system that binds to ligand-gated $\text{Na}^+$ and $\text{Ca}^{2+}$ channels to elicit EPSPs.

GABA

The principal inhibitory amino acid neurotransmitter in the brain that activates transmitter-gated $\text{Cl}^-$ channels to hyperpolarize postsynaptic neurons.

Dopamine

A biogenic amine neurotransmitter in the central nervous system that coordinates motor control and limbic pleasure pathways; its reuptake transporter is blocked by cocaine.

Substance P

An 11-amino-acid peptide neurotransmitter released by sensory neurons in the central nervous system in response to painful stimuli to transmit pain sensations to the brain.

Cortisol Receptor

A cytoplasmic nuclear receptor that undergoes a conformational change upon binding lipid-soluble cortisol, dimerizes, and translocates into the nucleus to regulate gene transcription.

Molecular Switch

An intracellular signaling protein that toggles between an inactive and active conformation in response to phosphorylation (kinases/phosphatases) or GTP binding (GEFs/GTPases).

Heterotrimeric G Protein

A membrane-associated signaling complex consisting of $\alpha$, $\beta$, and $\gamma$ subunits that dissociates into an active GTP-bound $\alpha$ subunit and a $\beta\gamma$ complex upon GPCR activation.

Adenylyl Cyclase

A plasma membrane enzyme activated by the $\alpha$ subunit of stimulatory G proteins ($\text{G}_s$) that catalyzes the synthesis of cyclic AMP ($\text{cAMP}$) from $\text{ATP}$.

Protein Kinase A (PKA)

A serine/threonine kinase whose catalytic subunits are activated when cyclic AMP binds to its regulatory subunits, initiating downstream phosphorylation cascades.

Epinephrine Signaling

A GPCR-mediated stress pathway in which adrenaline stimulates $\text{G}_s$, activates adenylyl cyclase, increases $\text{cAMP}$, and triggers PKA to activate glycogen phosphorylase while inactivating glycogen synthase.

Phospholipase C (PLC)

A membrane-bound enzyme activated by $\text{G}_q$ that cleaves phosphatidylinositol 4,5-bisphosphate ($\text{PIP}_2$) into membrane-bound diacylglycerol (DAG) and soluble inositol 1,4,5-trisphosphate ($\text{IP}_3$).

Inositol 1,4,5-Trisphosphate ($\text{IP}_3$)

A water-soluble second messenger released into the cytosol by PLC that opens ligand-gated $\text{Ca}^{2+}$ channels in the endoplasmic reticulum membrane.

Calmodulin

A ubiquitous cytosolic $\text{Ca}^{2+}$-binding protein that undergoes conformational changes upon binding calcium to activate downstream CaM-kinases and phosphatases.

Wnt Signaling Pathway

A developmental pathway where Wnt binding to the Frizzled receptor activates Disheveled to inactivate an APC destruction complex, allowing $\beta$-catenin to accumulate, enter the nucleus, and activate TCF target genes.

Receptor Tyrosine Kinase (RTK)

A single-pass transmembrane enzyme-coupled receptor that dimerizes upon ligand binding, stimulating cross-autophosphorylation of tyrosine residues on its cytoplasmic domains.

SH2 Domain

A specialized protein structural module that specifically recognizes and binds phosphorylated tyrosine residues on activated RTKs and intracellular adaptor proteins.

Ras Protein

A small monomeric GTPase anchored to the inner plasma membrane that acts as a molecular switch linking activated RTKs to the downstream MAP kinase cascade.

MAP Kinase Signaling Module

A sequential three-kinase phosphorylation cascade (MAP kinase kinase kinase $\rightarrow$ MAP kinase kinase $\rightarrow$ MAP kinase) that transmits proliferation and differentiation signals to the nucleus.

Sevenless

An RTK expressed on the surface of developing Drosophila R7 precursor cells that binds the Boss ligand on neighboring R8 cells to drive photoreceptor differentiation via Ras.

PI 3-Kinase / Akt Pathway

A cell survival signaling cascade activated by RTKs wherein PI 3-kinase generates $\text{PIP}_3$ to recruit and activate Akt, which phosphorylates Bad to release active anti-apoptotic Bcl2.

Notch Signaling

A contact-dependent cell-fate determination pathway where Delta binding triggers proteolytic cleavage of the Notch receptor by $\gamma$-secretase, sending the cleaved Notch tail to the nucleus.

Intermediate Filaments

Tough, unpolarized, rope-like cytoskeletal polymers ($10\text{ nm}$ in diameter) assembled from staggered antiparallel tetramers of coiled-coil dimers that protect animal cells against mechanical stress.

Desmosome

A cell-cell anchoring junction that mechanically links the keratin intermediate filament networks of adjacent epithelial cells through cytoplasmic plaque linker proteins and transmembrane cadherins.

Hemidesmosome

A cell-matrix junction that anchors intracellular keratin intermediate filaments of basal epithelial cells to the underlying basal lamina via transmembrane integrin proteins.

Epidermolysis Bullosa Simplex (EBS)

A genetic skin blistering disease caused by mutations in keratin genes that disrupt intermediate filament assembly, making basal epidermal cells susceptible to mechanical rupture.

Neurofilaments

Intermediate filaments found in nerve cell axons that form cross-linked bundles to provide longitudinal mechanical strength and maintain axonal caliber.

Microtubules

Hollow, polarized cylindrical cytoskeletal filaments ($25\text{ nm}$ in diameter) composed of 13 protofilaments of $\alpha$-tubulin (minus end) and $\beta$-tubulin (plus end) heterodimers.

Dynamic Instability

The rapid switching of microtubules between phases of elongation and sudden shrinkage (catastrophe), governed by the presence or loss of a stabilizing GTP cap at the plus end.

Centrosome

The major microtubule-organizing center (MTOC) in animal cells that anchors microtubule minus ends and coordinates the assembly and orientation of the mitotic spindle.

Kinesin

An ATP-dependent motor protein that moves processively toward the plus ($$+$) end of microtubules, driving anterograde transport of cargo away from the cell center.

Cytoplasmic Dynein

An ATP-driven motor protein that walks toward the minus ($-$) end of microtubules, directing retrograde transport of vesicles and organelles back toward the centrosome.

Axoneme

The core cytoskeletal engine of cilia and flagella featuring a characteristic $9 + 2$ arrangement of nine outer doublet microtubules linked around two central singlet microtubules.

Nexin

An elastic linking protein in cilia and flagella that cross-links adjacent outer doublet microtubules, converting dynein-driven sliding forces into bending movement.

Actin Filaments

Two-stranded helical cytoskeletal polymers ($7\text{ nm}$ in diameter) assembled from globular actin (G-actin) subunits that regulate cell shape, cell cortex structure, and crawling motility.

Actin Treadmilling

A steady-state dynamic equilibrium in which ATP-actin monomers continuously add to the plus end while ADP-actin monomers dissociate from the minus end at an equal rate.

Sarcomere

The fundamental contractile repeating unit of a muscle myofibril (approximately $2.5\,\mu\text{m}$ long), delimited by Z discs and composed of interdigitating actin and myosin filaments.

Myosin II

An actin-dependent motor protein composed of two ATP-hydrolyzing globular heads and an extended coiled-coil tail that forms bipolar thick filaments with a central bare region.

Sarcoplasmic Reticulum

A specialized smooth endoplasmic reticulum network in muscle fibers that sequesters and rapidly discharges $\text{Ca}^{2+}$ into the cytoplasm to initiate contraction.

Transverse (T) Tubules

Deep, inward invaginations of the muscle fiber plasma membrane that propagate action potentials into the cell interior, triggering $\text{Ca}^{2+}$ release from the sarcoplasmic reticulum.

Troponin-Tropomyosin System

An actin-associated regulatory complex wherein tropomyosin blocks myosin-binding sites at rest until troponin binds $\text{Ca}^{2+}$, shifting tropomyosin and exposing actin to myosin heads.